Iron(II) Complexes of 4-(Alkyldisulfanyl)-2,6-di(pyrazolyl)pyridine Derivatives. Correlation of Spin-Crossover Cooperativity with Molecular Structure Following Single-Crystal-to-Single-Crystal Desolvation

The complex salts [Fe(L1)2]X2 (1X2; L1 = 4-(isopropyldisulfanyl)-2,6-bis(pyrazolyl)pyridine; X– = BF4–, ClO4–) form solvated crystals from common organic solvents. Crystals of 1X2·Me2CO show abrupt spin transitions near 160 K, with up to 22 K thermal hysteresis. 1X2·Me2CO cocrystallizes with other, less cooperative acetone solvates, which all transform into the same solvent-free materials 1X2·sf upon exposure to air, or mild heating. Conversion of 1X2·Me2CO to 1X2·sf proceeds in a single-crystal to single-crystal fashion. 1X2·sf are not isomorphous with the acetone solvates, and exhibit abrupt spin transitions at low temperature with hysteresis loops of 30–38 K (X– = BF4–) and 10–20 K (X– = ClO4–), depending on the measurement method. Interestingly, the desolvation has an opposite effect on the SCO temperature and hysteresis in the two salts. The hysteretic spin transitions in 1X2·Me2CO and 1X2·sf do not involve a crystallographic phase change but are accompanied by a significant rearrangement of the metal coordination sphere. Other solvates 1X2·MeNO2, 1X2·MeCN, and 1X2·H2O are mostly isomorphous with each other and show more gradual spin-crossover equilibria near room temperature. All three of these lattice types have similar unit cell dimensions and contain cations associated into chains through pairwise, intermolecular S···π interactions. Polycrystalline [Fe(L2)2][BF4]2·MeNO2 (2[BF4]2·MeNO2; L2 = 4-(methyldisulfanyl)-2,6-bis(pyrazolyl)pyridine) shows an abrupt spin transition just above room temperature, with an unsymmetrical and structured hysteresis loop, whose main features are reversible upon repeated thermal scanning.


Experimental S4
Scheme S1 Synthesis of L 1 . S4 Table S1 Experimental data for the crystal structure determinations. S9 Figure S1 1 H and 13 C NMR spectra of L 1 . S14

Figure S2
The asymmetric unit and molecular packing in the crystal structure of L 1 . S15 Definitions of the structural parameters discussed in the paper S16 Chart S1 Angles used in the definitions of the coordination distortion parameters S and Q. S16 Chart S2 Definition of the Jahn-Teller distortion parameters q and f. S16

Figure S5
The asymmetric unit of 1[ClO4]2·Me2CO at 250 and 100 K. S19       Its asymmetric unit contains two complex dications, four perchlorate anions, one molecule of acetone which is ordered but only ca. two-thirds occupied, and one molecule of water. All these residues lie on general crystallographic sites. A full refinement of this crystal was obtained at 120 K. The isopropyldisulfanyl group S(18B)-C(22B) is disordered over three sites with an occupancy ratio of 0.50:0.30:0.20. Two of the four perchlorate ions are also disordered, one over two equally occupied orientations and the other over three sites with occupancies of 0.67:0.20:0.13. Although the acetone molecule is ordered, fixed distance restraints were required for this residue to attain a sensible geometry. All wholly occupied non-H atoms, S and Cl atoms with occupancies ≥0.5, and the partial acetone site, were modelled anisotropically. The water H atoms were not apparent in the Fourier maps and are not included in the refinements, but are accounted for in the formula of the crystal and the density calculation. The highest residual Fourier peak of +1.7 e Å −3 is 0.7 Å from Fe(1A).
A second dataset was collected from the same crystal at 250 K. It retained the same unit cell and space group at this temperature, but the dataset had a high mosaicity. A preliminary solution showed all four anions and at least two SiPr groups were disordered, and the occupancy of the lattice solvent sites was reduced. A full refinement was not achieved from this dataset, and this structure has not been deposited with the CCDC.

Structure refinements of 1[BF4]2•sf.
All the datasets of this compound were obtained from the same crystal. The compound is high-spin at 250 K, and the crystal retained its high-spin state the first time it was cooled to 100 K on the diffractometer. Rewarming the crystal to 250 K, then recooling it under nominally the same conditions, then afforded the crystal in its low-spin state. This behavior is consistent with the magnetic data from this material, which imply its thermal SCO occurs slowly on cooling below 140 K with a significant kinetic barrier. The different outcomes of the two experiments might be caused by small differences in the temperature ramp in the two experiments. Alternatively, they could reflect the introduction of additional defects or a reduction in domain size in the crystal following the first thermal cycle.
The asymmetric unit contains one formula unit of the compound, with all residues on general crystallographic sites. No disorder is present in either of the low temperature refinements. The 250 K dataset of this crystal had a high mosaicity, and a preliminary solution showed it to be highly disordered. A publishable refinement of that dataset was not achieved, and it has not been deposited with the CCDC. d The crystal remained in its high-spin state the first time it was cooled to 100 K on the diffractometer. e The same crystal transformed to its low-spin state when it was rewarmed, then cooled to 100 K a second time. S10 The 250 K dataset of this crystal had a high mosaicity, and a preliminary solution showed it to be highly disordered. A publishable refinement of that dataset was not achieved, and it has not been deposited with the CCDC. d The crystal remained in its high-spin state the first time it was cooled to 100 K on the diffractometer. e The same crystal transformed to its low-spin state when it was rewarmed, then cooled to 100 K a second time. S11 The 250 K dataset of this crystal had a high mosaicity, and a preliminary solution showed it to be highly disordered. A publishable refinement of that dataset was not achieved, and it has not been deposited with the CCDC. d The crystal remained in its high-spin state the first time it was cooled to 100 K on the diffractometer. e The same crystal transformed to its low-spin state when it was rewarmed, then cooled to 100 K a second time. S12 The 250 K dataset of this crystal had a high mosaicity, and a preliminary solution showed it to be highly disordered. A publishable refinement of that dataset was not achieved, and it has not been deposited with the CCDC. d The crystal remained in its high-spin state the first time it was cooled to 100 K on the diffractometer. e The same crystal transformed to its low-spin state when it was rewarmed, then cooled to 100 K a second time. S13 The 250 K dataset of this crystal had a high mosaicity, and a preliminary solution showed it to be highly disordered. A publishable refinement of that dataset was not achieved, and it has not been deposited with the CCDC. d The crystal remained in its high-spin state the first time it was cooled to 100 K on the diffractometer. e The same crystal transformed to its low-spin state when it was rewarmed, then cooled to 100 K a second time. Disulfide L 1 is most readily distinguished from its coproduct 4-(isopropylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (Scheme S1) by the Py H 3/5 singlet proton resonance, which occurs at 8.06 ppm in L 1 and at 7.73 ppm in the monosulfide in CDCl3. Color code: C, white or dark gray; H, pale gray; N, pale or dark blue; S, purple.

Definitions of the structural parameters in
VOh is the volume (in Å 3 ) of the FeN6 coordination octahedron in the complex, 7 which is typically <10 Å 3 in low-spin [Fe(bpp)2] 2+ (bpp = 2,6-di{pyrazol-1-yl}pyridine) derivatives and ≥11.5 Å 3 in their high-spin form. 8 S and Q are defined as follows: where bi are the twelve cis-N-Fe-N angles about the iron atom and gi are the 24 unique N-Fe-N angles measured on the projection of two triangular faces of the octahedron along their common pseudo-threefold axis (Chart S1). S is a general measure of the deviation of a metal ion from an ideal octahedral geometry, while Q more specifically indicates its distortion towards a trigonal prismatic structure. A perfectly octahedral complex gives S = Q = 0. 7,9 Because the high-spin state of a complex has a much more plastic structure than the low-spin, this is reflected in S and Q which are usually much larger in the high-spin state. The absolute values of these parameters depend on the metal/ligand combination in the compound under investigation, however. Typical values of these parameters for complexes related to [Fe(bpp R )2] 2+ are given in refs. 10 and 11.
Chart S1 Angles used in the definitions of the coordination distortion parameters S and Q.
The parameters in Chart S2 define the magnitude of an angular Jahn-Teller distortion, that is often observed in high-spin [Fe(bpp)2] 2+ derivatives like [FeL2] 2+ (q ≤ 90º, f ≤ 180 º). 11,12 They are also a useful indicator of the molecular geometry, in defining the disposition of the two ligands around the metal ion. Spin-crossover can be inhibited if q and f deviate too strongly from their ideal values, because the associated rearrangement to a more regular low-spin coordination geometry (q ≈ 90º, f ≈ 180º) cannot be accommodated by a rigid solid lattice. [12][13][14] In less distorted examples, significant changes in q and f between the spin states can be associated with enhanced SCO cooperativity. [15][16][17] Chart S2 θ and ϕ, used to discuss the structures of

Figure S4
The asymmetric unit of 1[BF4]2·Me2CO. Top, high-spin at 250 K; bottom, low-spin at 100 K. Other details as for Figure S3, which also shows the full atom numbering scheme for the structure. Color code: C, white; B, pink; F, cyan; Fe, green; N, blue; O, red; S, purple. S19 Figure S5 The asymmetric unit of 1[ClO4]2·Me2CO. Top, high-spin at 250 K; bottom, low-spin at 100 K. Other details as for Figure S3, which also shows the full atom numbering scheme for the structure. Color code: C, white; Cl, yellow; Fe, green; N, blue; O, red; S, purple.

Figure S8
Variable temperature unit cell volumes for 1[BF4]2·Me2CO, measured with cooling (black) and warming (gray) temperature ramps. Error bars are mostly smaller than the symbols on the graph.   Figure S7). That is not evident in this salt, however, where the spin transition is only clearly evident in the unit cell angle b.

Figure S9
Variable temperature unit cell dimensions for 1[ClO4]2·Me2CO (Table S5). Data were collected in cooling and warming temperature ramps. Thermal hysteresis in the spin-transition is visible in a, b and b.
Error bars are shown, but are mostly smaller than the symbols on the graph.   The disordered heterocycle core was modelled without restraints, but fixed S-S, S-C, C-C and 1,3-C···C distance restraints were applied to its peripheral isopropyldisulfanyl substituents.
The heterocyclic core of molecule B is not disordered in the 250 K structure of the same crystal, where that molecule is fully high-spin (previous page).

Figure S12
The asymmetric unit of 1[BF4]2•0.5Me2CO•0.5H2O. Top, at 250 K; bottom, at 120 K (the crystal is low-spin at both temperatures). The full atom numbering in the structure corresponds to that in Figure S3, with suffixes to distinguish between cations A and B in the model. Other details as for Figure S3.   Iron atom Fe(1B) and one of its L ligands are disordered at this temperature ( Figure S11). While the Fe−N distances are ambiguous, the bond angles and structural parameters show this disorder corresponds to separately resolved high-spin (74 % occupancy) and low-spin (26 %) forms of this cation site.

Figure S14
The asymmetric unit of 1[BF4]2·sf. Top, high-spin at 250 K; center, high-spin at 100 K; bottom, low-spin at 100 K. Other details as for Figure S3, which also shows the full atom numbering scheme for the structure. Color code: C, white; B, pink; F, cyan; Fe, green; N, blue; S, purple.  This compound is high-spin at 250 K. The crystal retained its high-spin form the first time it was cooled to 100 K, but transformed to its low-spin state when it was rewarmed and cooled a second time. That reflects a kinetic barrier to spin-crossover when the crystal is cooled too rapidly -see the main article for more details. b This crystal undergoes an abrupt spin-transition at T½ = 175 K with a thermal hysteresis width DT½ = 20 K. The crystal was measured in both its high-spin and lowspin forms at the same temperature inside the hysteresis loop, 170 K.  The S atom of this isopropyldisulfanyl group is disordered at this temperature.
The sum of the Pauling van der Waals radii of an S atom and an aromatic ring is 3.55 Å. 18 Hence, contact d corresponds to a strongly attractive intermolecular interaction in all these structures, but c is longer and may just corresponds to a van der Waals contact.  Figure 3.    (14) 102.212 (7) 3839.6(4) S41 Figure S18 Variable temperature unit cell dimensions for 1[ClO4]2·sf (Table S10). Data were collected in cooling and warming temperature ramps. Thermal hysteresis in the spin-transition is visible in a, b and b.
Error bars are shown, but are mostly smaller than the symbols on the graph. The acetone solvates of each salt in Figures S20 and S21 crystallize together, and were isolated by Pasteur separation for these measurements. Those samples had to be coated in nujol before measurement, to protect against solvent loss, which will contribute towards the lower signal-to-noise ratio in those powder patterns. Unlike the powder diffraction measurements, it was not possible to protect TGA samples against solvent loss. The absence of a significant mass loss below 200 °C implies these samples had lost their lattice solvent by the time the measurement was started. Hence, these are essentially TGAs of the 1X2•sf phases.

Figure S23
Magnetic susceptibility data for 1[BF4]2•sf, at a scan rate of 5 K min -1 (dark red) and 10 K min -1 (pale red). The 5 K min -1 data are the same as in Figure 1 of the main article.
The SCO becomes less complete, and the relaxation step is more pronounced, at the faster scan rate. That is characteristic of kinetic trapping of a fraction of the sample in their metastable high-spin form upon cooling. That has also been observed in some other [Fe(bpp R )2] 2+ derivatives showing abrupt spin transitions close to 100 K. 15,19 S45 Figure S24 Magnetic susceptibility data for a phase-pure sample of 1[ClO4]2•mMe2CO•0.5H2O. The data were measured in warming mode, at scan rate 5 K min -1 .
This sample was purified from a recrystallized sample of '1[ClO4]2•xMe2CO' by a Pasteur separation. The lack of any abrupt features between 150-170 K implies the material is phase-pure ( Figure S24). This is the only magnetic measurement we obtained from a purified acetone solvate phase, which is consistent with the spin state of its crystal structure.
The reversibility of the SCO was not examined in this measurement, so solvent loss might be contributing to the high temperature behavior. Consistent with that, the sample had decomposed when its powder pattern was measured after this experiment.
Be that as it may, the gradual SCO above 200 K is clear in these data, which is also consistent with the higher temperature behavior of the mixed-phase '1[ClO4]2•xMe2CO' material in Figure S25. The rapid loss of solvent before the measurement was a common problem with the acetone solvates in this work. Figure 2 shows the only time in four attempts that the clean conversion of 1[ClO4]2•Me2CO to 1[ClO4]2•sf was observed inside the magnetometer. We never did achieve this for 1[BF4]2•Me2CO.
No single crystals that are high-spin below 150 K were isolated from this mixture. Hence, the residual low-temperature magnetic moment in scans (i) and (ii) implies freshly prepared '1[ClO4]2•xMe2CO' might also contain a third phase. That could be related to 1[BF4]2•0.75Me2CO, which has a mixed spin-state population at low temperature.
Conversion of the whole sample to 1[ClO4]2•sf is complete in the fourth scan. The temperature of the hysteretic spin-transition in the final material is consistent with the phase-pure sample in Figure 2, and with the single crystal unit cell data from 1[ClO4]2•sf.

Figure S26
The asymmetric unit of 1[BF4]2·MeNO2 at 120 K. Other details as for Figure S3, which also shows the full atom numbering scheme for the structure. Color code: C, white; B, pink; F, cyan; Fe, green; N, blue; O, red; S, purple.

Figure S27
The asymmetric unit of 1[BF4]2·H2O at 120K. The full atom numbering in the structure corresponds to that in Figure S3. The H atoms on the lattice water molecule are included in the figure; other details as for Figure S3. Color code: C, white; B, pink; F, cyan; Fe, green; N, blue; O, red; S, purple.

Figure S28
The asymmetric unit of 1[ClO4]2·nMeNO2. Top, mostly low-spin at 250 K; bottom, fully lowspin at 120 K. Other details as for Figure S3, which also shows the full atom numbering scheme for the structure. Color code: C, white; Cl, yellow; Fe, green; N, blue; O, red; S, purple.

Figure S29
The asymmetric unit of 1[ClO4]2·MeCN. Top, mostly high-spin at 250 K; bottom, low-spin at 120 K. The full atom numbering in the structures corresponds to that in Figure S3, with suffixes to distinguish between cations A and B in the model. Other details as for Figure S3. Color code: C, white; Cl, yellow; Fe, green; N, blue; O, red; S, purple.    Table. Table S13 Intermolecular n···p contacts in crystal structures isomorphous with Figure S30. This S atom of this isopropyldisulfanyl group is disordered at this temperature.

S51
The sum of the Pauling van der Waals radii of an S atom and an aromatic ring is 3.55 Å. 18 Hence, contact a corresponds to a significant attractive intermolecular interaction in all these structures, but b is longer and probably just corresponds to a van der Waals contact. The measured mass loss for 1[ClO4]2·nMeNO2 is a good fit for its crystallographic composition with n = 0.9 (a whole equivalent of MeNO2 would give a mass loss of 6.4 %). The hydrate materials lose their water easily. However, like all the dried solvates, they also reabsorb that moisture quickly when exposed to air.   Figure S32). However, the powder pattern of 1[ClO4]2•MeCN does not match its single crystal phase, or the other solvates in this work. It may have undergone a phase change following solvent loss during sample preparation; or, the single crystal phase may simply be a minor contaminent of the bulk sample, which adopts another structure type. In any case, the crystal structures of 1[ClO4]2•MeCN clearly do not correspond to the structure of that bulk material.

Figure S34
Variable temperature magnetic data for the acetonitrile solvate materials. Data were collected on a 300⟶350⟶5⟶300 K temperature cycle, at a scan rate of 5 K min -1 .
Data for the nitromethane solvate and hydrate compounds, which are isomorphous with 1[BF4]2·MeCN, are in Figure 8 of the main article.
All the 1X2·MeCN, 1X2·MeCN and 1X2·H2O compounds are isomorphous by powder diffraction, except for 1[ClO4]2·MeCN ( Figures S31-S32). The data for all these compounds show good reversibility after warming to 350 K, implying they are not affected by in situ solvent loss during the measurement. That contrasts with the acetone solvates, which are more sensitive to solvent loss under the same conditions.

Discussion of the Hirshfeld surface analyses
A Hirshfeld surface is the boundary surrounding a molecule in a crystal, where the electron density from the enclosed molecule is equal to that from its nearest neighbors. 20 The surface can be plotted in various ways, including interaction (or fingerprint) maps which show intermolecular distances from each atom inside the surface (di, i = internal) and its nearest neighbours in the lattice (de, e = external). These are scaled according to their distance from the Hirshfeld surface about the residue of interest. 21  Each graph is marked with the Pauling Van der Waals radii of the elements plotted. 18 Only data points with di and de less than the relevant Van der Waals radius are significant intermolecular contacts. Strong interactions like O-H...X hydrogen bonds afford characteristic sharp lines on the donor X...H and acceptor X...H maps, projecting well below the Van der Waals radii of each element. Weaker interactions like C-H...X or anion...p appear broader in the maps, and extend only slightly below the Van der Waals radii limits.
A limitation is that the technique does not account for disorder. When one of the two interacting molecules is disordered, it leads to a broadening in the fingerprint map of the interaction, which may also appear artificially shortened. Such disorder is highlighted on the following Figures, where relevant.
Interaction maps are plotted for the isothermal high-spin and low-spin structures in 1[BF4]2·sf and 1[ClO4]2·sf, and for the other available crystal structures in this work. Three maps are plotted for each compound, which respectively highlight weak intermolecular C-H···Y (Y = F or O) contacts between the cations and anion in the lattice; weak anion···p interactions; and, the pairwise n···p contacts between cations involving their disulfanyl groups.
Data for two compounds allow the influence of temperature on these maps to be probed. Firstly, isothermal low-and high-spin data for 1[BF4]2·sf are plotted at 100 K, together with the 250 K high-spin structure. Secondly, the low-and high-spin structures of 1[ClO4]2·sf are both available at two temperatures, including isothermal analyses at 170 K.
While disorder at the higher temperatures complicates the comparison, the Hirshfeld fingerprints near the van der Waals radii for each spin state at different temperatures are essentially the same (Figures S36-S37). This is especially clear for 1[ClO4]2·sf, where the two high-spin structures exhibit essentially the same disorder regime ( Figure S37). Evidently, SCO has a greater effect on the Hirshfeld interaction fingerprints in these compounds, than the background contraction or expansion on the lattice between temperatures. 23 That is consistent with another of our recent studies, which drew the same conclusion. 24 More detailed interpretations of the intermolecular interactions in each structure are given beside the relevant Figure. The most important conclusion is that there are short, directional intermolecular interactions in 1X2·Me2CO and 1X2·sf that can explain their hysteretic SCO, in isolation. Rather, we attribute this SCO cooperativity to two factors: (i) The large angular structural rearrangements undergone by the cations during SCO, and; (ii) The large surface contact area between neighbor cations, which is a consequence of the pairwise n···p interactions linking them into chains. That will efficiently transmit the molecular structure changes through the lattice as the transition proceeds. The circled data points for the high-spin structures involve contacts to disordered ClO4ions or disulfanyl groups ( Figures S4-S5), which should be treated with caution.
The plots show short, directional C-H···Y (Y = F or O) interactions in the lattice, which extend significantly below the covalent radii of an H and F/O atom. Two contacts contribute to this: · C(33)-H(33)···F(46 ix ) [symmetry code (ix): x, 3  The pairwise intermolecular n···p interactions in Figure S30 are also clearly visible in the low-spin S···C fingerprint maps. These contacts are up to 0.18 Å shorter in the low-spin structures, while the highspin S···C maps are also broadened by disorder in some S atoms.

Figure S36
Hirshfeld fingerprint maps of 1[BF4]2·sf in its isothermal high-and low-spin states, showing intermolecular contacts surrounding the complex cation. Data from a 250 K high-spin structure are also included for comparison. See page S58 for more details.
The circled data points for the 250 K structure involve contacts to disordered BF4ions ( Figure S14), which should be treated with caution.
These data are discussed on the following page.

Figure S37
Hirshfeld fingerprint maps of 1[ClO4]2·sf in its isothermal high-and low-spin states, showing intermolecular contacts surrounding the complex cation. Data from the 110 K low-spin and 250 K high-spin structures are also included for comparison. See page S58 for more details.
The circled data points for the structures at T > 110 K involve contacts to disordered ClO4ions or disulfanyl groups ( Figure S15), which should be treated with caution. The long, thin tail on the low-spin C···Y interaction maps implies a weak but directional anion···p contact, which is not evident for the acetone solvate crystals. One anion 'Y' atom, F(46)/O(46), is indeed oriented towards the centroid of a pyridyl ring in 1[BF4]2·sf and 1[ClO4]2·sf. The Y···centroid distances of 2.9-3.0 Å imply this is simply a Van der Waals contact, however, which is why the interaction does not extend below the covalent radii in the plots.
The pairwise n···p interactions in Figure S16 are less clear in these plots. The closest intermolecular S···C contacts in 1[BF4]2·sf and 1[ClO4]2·sf are ca 0.2 Å longer than in the acetone solvates, when those S atoms are crystallographically ordered (Table S8). That should not impact the ability of these contacts to mediate mechanical coupling between the cations during the cooperative spin-transitions, however. The circled data points involve contacts to disordered anions or disulfanyl groups ( Figures S26-S28), which should be treated with caution.
There are no notably short, directional intermolecular contacts, which are not contaminated by disorder, in any of these fingerprint maps.

Figure S40
The asymmetric unit of 2[BF4]2·0.5MeNO2. Top, at 290 K; bottom, at 100 K. The crystal is lowspin at both temperatures. Other details as for Figure S39, which also shows the full atom numbering scheme for the structure. Color code: C, white; B, pink; F, cyan; Fe, green; N, blue; O, red; S, purple.
Anion B(40)-F(44) is fully occupied on a general crystallographic site, but B(45)-F(49) lies near the crystallographic inversion center, and is only half-occupied by symmetry. The remaining half-anion equivalent B(50A)-F(54A), is superimposed upon a half-solvent molecule C(50B)-O(53B) occupying the same lattice site. The disordered region occupies a cavity spanning the inversion center 1, 0, 0, with each cavity containing one anion and one solvent molecule in a disordered distribution about that inversion center.
The composition of the mixed anion/solvent site is obvious in the 100 K structure, where each individual residue is crystallographically ordered. The disorder at 290 K makes this less clear, but the refinement is consistent with the 100 K model and is crystallographically reasonable.

Figure S41
The asymmetric unit of 2[BF4]2·0.5MeCN at 120K. Other details as for Figure S39, which also shows the full atom numbering scheme for the structure. Color code: C, white; B, pink; F, cyan; Fe, green; N, blue; S, purple.
The powder pattern is quite broad, but the agreement with the simulation is excellent.
The feature near 200 K first appears on the cooling branch of scan 1, but is present in both warming and cooling modes for scans 2-4. It also slowly grows in on repeated scanning. We proposes this feature arises from slow desolvation of the sample as the experiment proceeds. The curve plateaus near 160 °C at -6.0 % mass loss, which corresponds to ca 0.8 equiv MeNO2. However, the expansion shows an inflection in the curve at 115 °C/3.3 % mass loss, close to the expected value for the crystallographic solvent content as shown in the Figure. The origin of the additional mass loss is uncertain. There were no crystalline contaminents in the sample ( Figure S42), and no significant void space in the crystal structure that could accommodate extra solvent. One possibility could be partial decomposition of the BF4ions, which is usually expected above 200 °C but can occur at lower temperatures in some circumstances. 25 Be that as it may, the material is stable to solvent loss below 68 °C (340 K), and only loses solvent slowly above that temperature. That is consistent with our suggestion that the feature near 200 K in the magnetic measurements, which grows in slowly upon repeated thermal scanning, arises from slow solvent loss from the sample ( Figure S43).  These parameters were derived by fitting the data in Figure S46 to eq 1 and 2, where nHS(T) is the high-spin fraction of the sample at temperature T: ln[(1-nHS(T))/nHS(T)] = ΔH/RT -ΔS/R (1) ΔS = ΔH/T½

S69
(2) The thermodynamic parameters in the Table are typical for a complex of this type. 26 Our previously published correlation predicts T½ = 229 K for [Fe(bpp R )2] 2+ derivatives with an alkyldisulfanyl 'R' substituent, 26 whose sP Hammett parameter is +0.13. 27 While correlations of T½ against the alternative sP + Hammett parameter are often more accurate, 26 sP + values for alkyldisulfanyl substituents have not been measured. 27